The chronopotentiometric method for measuring the kinetics of metal dissolution and cementation reactions

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The validity of the chronopotentiometric method, which has been used for studying the kinetics of metal dissolution and cementation (displacement) reactions was examined. Using the dissolution of electrodeposited zinc films as an example, results are presented for the dissolution in a triiodide solution, in an acid solution and in an acidified triiodide solution. The effect of the substrate electrode, on which the zinc film is deposited, was found to be negligible in the case of the triiodide dissolution reaction which is known to be diffusion controlled. However, the substrate electrode can have a profound effect on the dissolution in acid, a reaction which is known to be chemically controlled. In addition, the apparent dissolution rate in acidified triiodide solutions can be greater than the sum of the separate dissolution rates. Explanations for these findings, based on the catalytic effect of the substrate electrodes, are presented. The applicability of the chronopotentiometric method to the cementation of copper (II) on to electrodeposited zinc filsm was also investigated. Similar difficulties to those found with the zinc/acidified triiodide system were encountered. As a result of the understanding of the pit-falls of the chronopotentiometric method developed in this work, it was possible to review critically previous measurements of this type, which have been reported in the literature. It was concluded that, while the majority of previous studies are correct, a serious error has been made in one paper.